Kinetics and Mechanism of the Hydrolysis of Acyclic Oxyphosphonium Salts

Article abstract of DOI:10.1021/ja00304a029

A kinetic study is reported of the hydrolysis of seven acyclic alkoxyphosphonium ions <4-X-C6H4(Me)P(OMe)2(1+) (X=H, Cl, Me), PhP(OR)3(1+) (R=Me, Et), Ph2P(OMe)2(1+), Ph2(Me)P(1+)OMe> and two alkoxyaryloxyphosphonium ions <4-XC6H4(Me)P(OMe)OC6H4-4-OMe(1+) (X=H, OMe)>.First-order rate constants follow the relationship k_obsd=k₀+k_OH+k_A, showing a reaction with water, hydroxide, and the base component of the buffer.The last two reactions are shown or argued to involve only P-O cleavage in the hydrolysis.The water reaction occurs with some C-O cleavage as well.The mixed phosphonium ions hydrolyze over a range of acidities to produce 4-methoxyphenol and the methyl phosphinate ester.The P-O cleavage reaction in all cases is argued to involve rate-limiting formation of a pentavalent hydroxyphosphorane intermediate.The buffer reaction represents general base catalysis of the addition of water, the base assisting the water attack by simultaneously removing a proton.Broensted coefficients are large, ranging from 0.6 to 0.85.This implies a transition state with a considerable amount of proton transfer.A comparison is presented with the general base-catalyzed hydration reactions of carbocations, which show significantly smaller β values.The water rates for the phosphonium ions lie close to the Broensted plots, suggesting that this reaction is also occuring with general base catalysis.Large rate decreases are observed with added acids and salts.These effects are explained by the general base mechanism with considerable hydronium ion character in the transition state.A comparison is also presented of the effects of sulfuric acid on the rates of hydrolysis of an alkoxyphosphonium ion and an analogous hydroxyphosphonium ion or protonated phosphinate ester.